This invention relates generally to processes for recovering substantially pure titanium metal sponge from rutile ore and more particularly, provides a single vessel self-replenishing electrolytic process and apparatus for obtaining substantially pure titanium metal sponge.
Most titanium metal has been produced from its ores by the chlorination of its oxide-like rutile ore; the separation and purification of the TiCl.sub.4 produced; the reduction of the TiCl.sub.4 to titanium metal by use of an active metal such as magnesium, sodium or calcium and electrolytically recovering the reducing metal and chlorine, both for recycling.
Another well known reduction process for obtaining titanium metal from its ore is the Kroll process represented by U.S. Pat. No. 2,205,854.
The Kroll process involves the reaction of the titanium tetrachloride with molten magnesium metal in an inert gas atmosphere at normal or atmospheric pressure. A refractory metal is employed as a lining for the steel reaction vessel or crucible, the refractory metal being oxidized to prevent diffusion of the lining into the steel. The magnesium metal is heated to 800.degree. C. and titanium tetrachloride dripped onto the metal. The product is solid titanium metal and liquid magnesium chloride. Separation of the metal requires further tedious and expensive processing.
Winter U.S. Pat. No. 2,890,112 discloses another method for producing titanium metal by reduction of the tetrachloride wherein both the magnesium and chlorine are recovered and recycled. Winter suggests the use of sodium metal produced by electrolysis of a ternary salt along with a molten calcium-magnesium alloy and chlorine gas. The chlorine gas is reacted with a titanium ore/coke mixture. The reduction reactive does not employ magnesium alone. A process involving molten sodium often is dangerous and expensive.
Glasser et al U.S. Pat. No. 2,618,549 provides a method for production of elemental titanium from its ores by reduction of a titanium halide such as titanium tetrachloride by means of an alkali metal amalgam such as is produced in mercury-amalgam chlorine cells used in the production of caustic soda. Chlorine produced in the electrolysis is employed to produce the titanium tetrachloride from the oxide ore.
The amalgam reactant is separated after its production and added to a reaction vessel along with the titanium tetrachloride. The reduction reaction is carried out in the presence of an inert gas and with vigorous agitation of the reactants. When the reaction is completed, the product is transferred to a separating furnace without exposure to air using gravity or other feed means. The product is first distilled to drive off mercury and thereafter, the residual material is heated to above 1500.degree. F. to separate sodium chloride. Subsequent purification steps are suggested. Not only does the Glasser process require the use of inherently dangerous mercury and sodium, it is expensive, requires much equipment and multiple steps as well as proximity and access to caustic soda processing plants.
Other processes are described in Maddex, U.S. Pat. No. 2,556,763; Blue, U.S. Pat. No. 2,567,838; Winter, Jr., U.S. Pat. No. 2,586,134; Winter, Jr., U.S. Pat. No. 2,607,674; Winter, Jr., U.S. Pat. No. 2,621,121; Loonam, U.S. Pat. No. 2,694,653 and Dietz, U.S. Pat. No. 2,812,250.
Thus it is noted that the process of obtaining titanium metal from rutile ore by reduction thereof is well known. The conversion of the ore, which is in the form of an oxide, to the tetrachloride and reaction of the tetrachloride with magnesium metal to form magnesium chloride and titanium metal has been discussed. Titanium produced in this manner required considerable purification, both from impurities originating from the ore as well as removal of magnesium chloride trapped within the product and any remaining magnesium metal. Known processes often included electrolytic processes where the production of magnesium metal was electrolytic. Magnesium was produced electrolytically isolated, removed from the cell and purified. The purified magnesium metal then was remelted for the reduction reaction with the titanium tetrachloride. These processes were unduly expensive in terms of the energy requirements, the precautionary expedients required for safety (especially in view of the reactivity of magnesium and the need to separate and transfer same), the number of steps and cost of purification, the waste of raw materials and loss in both transfer and purification.
Of all the problems inherent in the conventional procesess for reduction of titanium ore with magnesium metal, exposure of the magnesium metal to air was the most serious, particularly in terms of safety.
Conventionally, processes for producing titanium sponge include draining of the magnesium chloride from the reaction cell as the reduction reaction proceeds. The drained magnesium chloride is recycled by electrolysis producing magnesium metal, which is drawn from the cell and cast as ingots. These ingots then are placed in a steel retort and melted. The resulting molten magnesium is either transferred to another vessel for reaction with titanium tetrachloride or the molten magnesium charge reacted in the same vessel in which it was melted.
The reduction reaction of titanium tetrachloride with magnesium is exothermic, with the by-product, magnesium chloride, heated to about 1400.degree. F. Thus the magnesium chloride is molten. Conventionally, the heat released in the exothermic reaction is lost, that is, not advantageously used.
Another problem encountered with conventional processes is the establishment of access both to the titanium metal produced, and to the magnesium chloride by-product. The reaction vessel conventionally has to be cooled. After cooling, operators using physical means such as jack-hammers or the like must literally break up the solidified material. The titanium metal resulting must be refined for obtaining the purity commercially desired. A further danger encountered is the production of harmful amounts of phosgene gas resulting from the reaction of impurities in the ore with chlorine produced, or with the magnesium chloride, under the elevated temperatures of the reactions.
The production of titanium metal is a very precise operation since the hot metal combines with oxygen, nitrogen and moisture of the air. The metal also combines with carbon and most construction metals. Refractory materials also are vulnerable to attack due to their oxygen content. Contaminants produced render the resulting metal so hard and brittle that it is useless for most applications. If once picked up, there appears to be an absence of practical methods for removing such impurities. If one carries out the reaction under an atmosphere of an inert gas, such as helium or argon at around 800.degree. C., the titanium alloys with the iron in the vessel to a minimal extent. Nevertheless, the metal layer in contact with the vessel wall contains too much iron to meet specifications and must be discarded.
In operation under conventional processes, the magnesium ingots are pickled in dilute acid to remove surface oxidation followed by rinsing and drying steps. The dried magnesium charge is placed in a cylindrical flat-bottomed steel pot. The cover is welded in place, the vessel tested for leaks and, if no leaks are detected, all the air is removed from the vessel by evacuation, followed by release of the vacuum repeatedly with helium or argon. The vessel, with the magnesium charge, is placed in a vertical cylindrical furance and heated either by electricity or by fuel combustion. As soon as the magnesium begins to melt, purified titanium tetrachloride is introduced at a carefully controlled rate. Inert gas pressure within the vessel is maintained to prevent inward air leakage. The rate of introduction of the titanium tetrachloride is controlled such that excess heat generated in the vessel is dissipated through the vessel walls, external heat being unnecessary to maintain the vessel temperature between 750.degree. and 1000.degree. C.
Accordingly, improvements are sought in the recovery and in the purification of the recovered product with reduction in the danger, lowering of the costs, especially energy costs, increase in product purity and access to the desired product.
The less handling the better and the less exposure of the reactants are important criteria to be met.